The strategy for implementing damage detection and characterization of mechanical structures is commonly called Structural Health Monitoring (SHM). Damage is defined as modifications of the material and/or of the geometrical properties of a structural system, comprising modifications of boundary conditions and connections of the system, that worsen performance of the system. The SHM process implies the observation of the mechanical system during the time using periodically: measurement of dynamic responses coming from an array of sensors, extraction of data of damage characteristics sensed from these measurements, and statistical analysis of these data of characteristics for determining the present health state of the system (also called structural analysis).
This process provides information about the capacity of the structure for carrying out its function, considering the unavoidable aging and degradation in working environments. After extreme events, such as earthquakes or explosions, the SHM is used for a quick screening of the conditions of the structure for providing, almost in real time, reliable information about the integrity of the structure itself.
Nowadays, SHM systems use sensors placed on the surfaces to be controlled. For example, sensors used (anemometers for calculating the wind speed, accelerometers, extensometers, motion transducers, temperature sensors, sensors for detecting motion of weights, etc.) for monitoring bridges are placed on the external surfaces of beams, wire ropes or pillars, in order to: estimate the effects of loads on the bridge, evaluate the weakening of the bridge, and/or foresee the probable evolution of the bridge and its expected lifetime.
SHM systems with sensors adapted to be buried in building structures to be monitored have been devised. These sensing devices include sensors (of pressure, humidity, temperature, etc.) that have at least one remote powering and transmission antenna for transmitting the measured values outside of the block of building material, as in RFID devices (that are sensorless) illustrated in the article by A. Finocchiaro, G. Ferla, G. Girlando, F. Carrara e G. Palmisano, “A 900-MHz RFID System with TAG-Antenna Magnetically-Coupled to the Die”, 2008 IEEE Radio Frequency Integrated Circuits Symposium, pages 281-284. This kind of sensing devices are disclosed, for example, in US patent application No. 2009/0033467 and in PCT WO 2012/084295, herein incorporated by reference, and are depicted in FIGS. 1, 2, 3 and 4.
As shown in FIG. 3, the sensor 10 is integrated in a chip IC 1 made of a semiconductor material, is supplied and communicates in a contactless fashion because it is electromagnetically coupled (for example, inductively, or capacitively or through an antenna) with an electric communication line with the outside, as shown in FIGS. 1 and 2. With this technique, sensors buried in the building material may be supplied from the outside and remain galvanically isolated from the respective communication lines with the outside.
Sensing device having sensors electromagnetically coupled to respective communication lines 2, eventually ending with an antenna 22 and equipped with elements of electromagnetic expansion, are shown in FIGS. 3 and 4. Their structure and how they are used is described in detail in PCT application WO 2012/084295. They may be tied to a support 211 buried in a block of building material for sensing at least one characteristic of the building material along the whole block or at least in a portion thereof.
In larger building structures, such as, for example, in pillars of bridges, it may be important to sense the distribution of forces in the structure. To this end, the devices disclosed in the documents US 2009/0033467 and/or WO 2012/084295 are buried inside the structure to remain oriented along three coordinate reference axis, for measuring the three orthogonal components (or in any case according to three different axis) of the forces in the structure.
These pressure sensors are typically provided by an integrated semiconductor circuit with a sensing surface, eventually passivated to be adapted to be placed in direct contact with the building material, that senses a pressure value in a direction orthogonal thereto.